[unreadable] The doubly labeled water (DLW) method of measuring total energy expenditure (TEE) has emerged as a vital tool in obesity research. It has enabled researchers to assess human energy requirements in health and in disease, to study the etiology of obesity, and to validate tools for the assessment of dietary intake. Picarro proposes to develop novel instruments that will measure oxygen and hydrogen isotope ratios in water with high precision by cavity ringdown spectroscopy (CRDS). As compared with existing mass spectrometery techniques, the proposed instruments will not only lower the costs of DLW studies but also enable physicians and medical technicians to measure TEE of individual patients by the DLW method in clinical settings. In Phase I, Picarro will build an isotopic analyzer in order to identify the appropriate isotopic water absorption lines with high resolution, and demonstrate detection sensitivity of the H2O18 line in water with a signal to noise ratio of better than 1 part in 5000. In phase II, Picarro will develop compact prototypes suitable for deletion of both oxygen and hydrogen isotopes and compare their performance with conventional mass spectrometry in a clinical study. A fieldable instrument not requiring extensive sample preparation would greatly extend the applications reach of energy expenditure methodology. The instrument could also be used by manufacturers of heavy water for quality control of their product. [unreadable] [unreadable] [unreadable]